It has long been known to use various surface modification techniques including glow discharge plasma to change the surface characteristics of polymeric materials. For example, it may be advantageous to improve the bondability of an implantable polymeric medical device or to change the wettability of a polymeric fabric. Fluorocarbons have frequently been used as both the surface modified substrate materials and as materials used to modify surfaces of other polymeric substrates.
These surface modifications can take several forms. Plasma polymerization by radio frequency gas plasma using polymerizing gases can polymerize a new material onto the surface of another substrate. Unsaturated fluorocarbon gas plasmas can be used, for example, to polymerize a fluorocarbon layer onto a polystyrene substrate. Alternatively, plasma activation is used with non-polymer forming gases such as oxygen or saturated fluorocarbons to chemically modify a substrate surface. The plasma activation of a fluoropolymer substrate surface with oxygen gas, for example, can result in the replacement of fluorine atoms from the substrate surface with oxygen in order to enhance the wettability of that surface. Still another technique is plasma cleaning or plasma etching where a reactive gas plasma is used to etch or roughen a surface by removing quantities of the substrate material comprising the surface. This can be done for surface cleaning or for increasing bondability, for example. Etching can also be accomplished with other energy sources such as ion beams. Masking techniques can be used to selectively etch portions of a surface to produce a desired pattern. Alternatively, specific surface patterns can be produced in polymeric surfaces by molding techniques well known to those skilled in the art.
Plasma polymerization, plasma activation and plasma etching are all considered to be specific types of plasma treatment but are not considered as mutually exclusive categories. The plasma etching of some substrates to enhance bondability is, for example, sometimes called plasma activation.
An article in Medical Product Manufacturing News (Using Gas Plasma to Re-engineer Surfaces, Nancy B. Mateo, September/October 1990), provides a general description of known gas plasma surface modification methods. The author states that increasing surface wettability and adhesiveness are among the most routine uses for gas plasmas.
U.S. Pat. No. 4,919,659 to Horbett, et al., teaches the modification of bio-material surfaces by radio frequency plasma in order to enhance the growth of cell cultures on the bio-material surfaces. The modification involves the plasma polymerization of overcoat layers onto a surface of an implantable bio-material.
A paper by A. M. Garfinkle et al., ("Improved Patency in Small Diameter Dacron Vascular Grafts After a Tetrafluoroethylene Glow Discharge Treatment," presented at the Second World Congress on Biomaterials 10th Annual Meeting of the Society for Biomaterials, Washington, D.C., Apr. 27-May 1, 1984) describes the use of plasma polymerization with the monomer TFE gas to modify the luminal surface of Dacron vascular grafts by depositing thereon a coating of tetrafluoroethylene.
An article by C. Tran and D. Walt (Plasma Modification and Collagen Binding to PTFE Grafts. Journal of Colloid and Interface Science, Oct. 15, 1989, vol 132 no. 2, pp 373-381), describes the use of RF and electrical glow discharge plasma deposition systems to clean and coat the luminal surface of porous expanded polytetrafluoroethylene GORE-TEX.RTM. Vascular Grafts. Cleaning was done with argon plasma for one hour followed consecutively by plasma polymerization with hexane and anhydrous ammonia for one hour each. The grafts were then coated with collagen. Wettability of the plasma modified polytetrafluoroethylene (hereinafter PTFE) surface was found to be increased. Y. S. Yeh et al. (Blood Compatibility of Surfaces Modified by Plasma Polymerization. Journal of Biomedical Materials Research 1988 22;795-818) used rf gas plasma in a hexafluoroethane/H.sub.2 atmosphere to polymerize the surface of GORE-TEX Vascular Grafts. They described the surface morphology of the treated graft surfaces as being indistinguishable from untreated graft surfaces.
Y. Iriyama et al. (Plasma Surface Treatment on Nylon Fabrics by Fluorocarbon Compounds. Journal of Applied Polymer Science 1990 39;249-264) plasma polymerized or alternatively plasma activated nylon fabrics with low temperature fluorocarbon plasmas so as to increase the hydrophobicity of these fabrics. They found water droplet roll-off angle to be a better indicator of rough surface hydrophobicity than measurements of water droplet contact angles. A good description of the method of making water droplet roll-off angle measurements is provided.
In U.S. Pat. No. 4,946,903, J. Gardella et al., teach plasma activation of fluoropolymers with radio frequency glow discharge to increase the wettability of their surfaces. This is accomplished by substituting hydrogen and oxygen or oxygen-containing radicals for fluorine atoms in the surface of the fluoropolymer. Porous expanded PTFE was used as an example fluoropolymer.
M. Morra et al., (Contact Angle Hysteresis in Oxygen Plasma Treated Polytetrafluoroethylene, Langmuir 1989 5;872-876; Surface Characterization of Plasma-Treated PTFE, Surface and Interface Analysis 1990 16;412-417), exposed non-porous PTFE surfaces to both oxygen and argon gas plasmas. With oxygen plasmas they found that 15 minute treatments produced extensive plasma etching of the surface while argon treatment for the same time did not alter surface smoothness. The argon treated surfaces were found to be more hydrophilic than the untreated precursor material. Morra also described that the roughened surface resulting from oxygen plasma treatment showed increased hydrophobicity as a direct function of the increased surface roughness, with water advancing contact angles up to 166 degrees.
U.S. Pat. No. 4,933,060 to Prohaska, et al., teaches the plasma modification of a fluoropolymer surface by treatment with reactive gas plasma comprising primarily water, in order to increase the adhesive bondability of such surfaces. The surfaces are rendered hydrophilic, apparently by the defluorination and oxidation of the surface.
U.S. Pat. No. 4,064,030 to J. Nakai et al., describes the modification of molded non-porous articles of fluorine resin by sputter etching with ion beams in order to provide better adhesion. They state that their treated surfaces have superior adhering properties not attainable with conventional glow discharge treatment. Nakai et al., noted that wettability of a surface can be modified by varying treatment time, discharge power or chamber pressure, however no modified surfaces were described as being more hydrophobic than untreated PTFE having contact angles up to about 120 degrees.
An article by S. R. Taylor, et al., "Effect of Surface Texture On The Soft Tissue Response To Polymer Implants," Journal of Biomedical Materials Research 1983 17;205-227, John Wiley & Sons, Inc., describes ion beam etching by sputtering of non-porous PTFE surfaces. A modified textured PTFE surface having conical projections was produced wherein the projections had a mean height of about 12 microns, a mean base width of about 4 microns and a mean tip radius of about 0.1 micron. Little or no apparent chemical changes in the modified surface were detected. When implanted in a living body, these modified PTFE surfaces produced fibrous capsules of only 30 percent of the thickness of fibrous capsules produced by unmodified PTFE surfaces. The modified surfaces also demonstrated increased cell adhesion. Contact angle measurements were used to determine the surface energy of the modified PTFE surfaces, however, no results of surface energy analysis and no contact angle data were provided for the modified textured PTFE surfaces because of wicking of the diagnostic liquids on those surfaces.
G. L. Picha et al., ("Ion-Beam Microtexturing of Biomaterials," Medical Device and Diagnostic Industry, vol. 6 no. 4, April 1984), describe the manufacture of textured surfaces in non-porous PTFE and polyurethane by etching surfaces with ion-beams, with and without the optional use of sputter masks, for the purpose of increasing bondability.
U.S. Pat. No. 4,955,909 to Ersek et al., describes textured silicone surfaces for implantable materials wherein the surfaces comprise a series of formed pillars with valleys disposed between them. The textured surface is produced by thrusting specifically selected molecules against a non-porous silicone rubber surface with sufficient impact to produce pillars or projections of 20 to 500 micron size.
U.S. Pat. Nos. 4,767,418 and 4,869,714 to Deininger et al., describe a male mold useful for making tubular vascular grafts, the surface of the mold comprising a series of pillars. The basis for the mold is created by sputter-coating a layer of gold film onto the surface of a PTFE cylinder. The pillars are then formed by selectively photoetching the sputter-coated gold film with the aid of a masked photoresist.